Exposure and printing device

ABSTRACT

An exposure device in which a row of LED&#39;s is provided in the form of parallelograms having unequal sides, in which each of the corner points of one short side of a light-emitting surface is always in line, as considered in a direction perpendicular to the row, with one of the corner points of the other short side of an adjacent light-emitting element, and a printing device in which said exposure device is used.

FIELD OF THE INVENTION

The present invention relates to a method and means for providing auniform exposure for printing, and in particular to a printing meanscomprising a single row of light-emitting elements which are constructedin the form of parallelograms of unequal lengths with the shorter sidesbeing perpendicular to the image-wise exposure and the corners of theshort side being aligned with corners of the opposite short side of anadjacent light-emitting element.

BACKGROUND OF THE INVENTION

It is well known to use light-emitting elements, such as LED's, inprinting devices, see U.S. Pat. No. 4,780,731 and European PatentApplication A-0189664. Typically, two rows of LED's are used to achievea uniform exposure, see NL-A 8300111. However, using two rows requiresspecialized synchronization between the data for the first and secondrows.

The use of a single row of light-emitting elements has resulted innonuniform exposures. See, for example, Japanese Application 60-99672,63-309476 and 63-57262.

In U.S. Pat. No. 4,553,148, there is disclosed an exposure devicecomprising one straight row of light-emitting elements disposed withfixed spacing b between them in one plane. In this case the LED's aredisposed in a straight row to image, by means of a Selfoc array, onto amoving photoconductor. In order to increase the light Yield, the LED'sare constructed in the form of elongate rectangles or parallelograms. Bydisposing a cylindrical lens between the row of LED's and thephotoconductor, the elongate LED's are imaged as small squares ordiamonds.

A disadvantage of these known devices is that the light distribution onthe photosensitive medium as considered in the direction of the row isnot equal, and between two image dots there is a zone where there isdistinctly less light. To enable the photoconductor to be exposed atthese places too, the total light level has to be increased, e.g. byincreasing the LED's energization current, and this increases the heatdevelopment and reduces the LED life.

Accordingly, it is an object of the present invention to provide a meansfor achieving a uniform exposure using a single row of light-emittingelements.

SUMMARY OF THE INVENTION

Generally, the present invention overcomes the disadvantages normallyinherent in single row LED sources (LED'S). According to the inventionthe disadvantages of the prior art are overcome by means of an exposuredevice, in which all arbitrary imaginary strips of equal width situatedin the plane of the photoconductor and extending perpendicularly to therow always contain an equally large area that is exposed by thelight-emitting elements or LED'S. This is achieved by means of anenergization device for energizing the light-emitting elements in suchmanner that each zone of the photoconductor receives exactly the sameamount of light in the exposed areas.

More particularly, the present invention provides an exposure devicewhich comprises a plurality of light-emitting elements aligned in a rowin spaced apart relationship. The distance between elements is definedherein as "b." Each of the light-emitting elements is configured in theshape of a parallelogram having two long parallel sides positionedgenerally in the direction of exposure and two parallel short sides onthe leading edge and trailing edge, respectively, of the image. Thecorners of the leading short edge of one element are disposed to alignwith corresponding corners of the trailing edge of an adjacent element.

Consequently, for a given printing speed it is possible to use a minimumenergization current and non-exposed zones which would otherwise causedark strips in the direction of transit of the photosensitive medium areeffectively obviated in the print.

These and other advantages will be apparent from a perusal of thefollowing description of presently preferred embodiments of theinvention taken with reference to the drawings:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a printing device using a row of LED's.

FIG. 2a is a top plan view of a row of LED's according to the prior art.

FIG. 2b shows the light distribution associated with FIG. 2a.

FIG. 3a is a top plan view of an exposure device according to theinvention.

FIG. 3b shows a light distribution associated with FIG. 3a.

FIG. 4 shows a number of image dots formed with an exposure deviceaccording to FIG. 3a.

FIG. 5 is a top plan view of another exposure device according to theinvention.

FIG. 6 shows a number of image dots on the photosensitive mediumproduced by means of an exposure device according to FIG. 5.

FIG. 7 is another embodiment of an exposure device according to theinvention, and

FIG. 8 shows an image dot obtained with an exposure device according toFIG. 7.

PRESENTLY PREFERRED EMBODIMENTS

FIG. 1 is a diagram showing a printing device in which anelectrophotographic belt 11 is trained about three rollers 12, 13 and 20in the direction of arrow 14 (auxiliary scanning direction). A belt ofthis kind provided, for example, with a zinc oxide layer or an organicphotosensitive layer, is charged in known manner by means of a chargingunit 1 and then exposed image-wise. Those places which have not receivedany light are developed with toner powder by means of developing device2. The resulting powder image is transferred in known manner to a heatedsilicone rubber belt 3. A sheet of receiving material is fed from asheet tray 6 between rollers 4 and 5, the powder image being transferredfrom the silicone rubber belt 3 to the receiving sheet and fusedthereon. The resulting print is deposited a receiving tray 7. Anexposure device 19 comprises a substrate or support 15 having a row ofLED's 16 thereon extending perpendicularly to the direction of advance14 of the belt 11 above the latter.

An array of imaging glass fibres 17, also known as a Selfoc array isdisposed between the exposure device 19 and the belt 11 and images eachLED on the electrophotographic belt 11 with an imaging ratio of 1:1(position 18). An image signal is fed via line 23 to an energizingdevice 22. A pulse disc is disposed on the shaft of roller 13 anddelivers a signal in proportion to the movement of belt 11. This signalis fed to a synchronization device 21 in which a synchronization signalis generated. In response to the synchronization signal the imagesignals are fed to exposure device 19 so that the electrophotographicbelt 11 is exposed image-wise line by line to form a row of image dotson belt 11.

FIG. 2a is a diagrammatic top plan view of an LED array as known, forexample, from U.S. Pat. No. 4,553,148. A number of parallelogram-shapedLED's 24-28 are disposed in a row in a spaced apart relationship onsupport 15. A photosensitive medium passes the row of LED's in thedirection of arrow 14 (auxiliary scanning direction). The axis 29 alongwhich the row of LED's extends indicates the main scanning direction.

The light distribution forming in a plane through the main scanningdirection and perpendicular to the drawing plane is shown in FIG. 2b. Inthis graph the x-axis is drawn to the same scale as that of the row ofLED's in FIG. 2a. Thus, point 31 of light distribution curve 34 in FIG.2b indicates the amount of light received by a point on thephotosensitive medium if that point is fed in the auxiliary scanningdirection 14 over the broken line 30 along the row of LED's in FIG. 2a.The amount of light received by a point on the photosensitive medium fedalong line 32 past the row LED's is also denoted by point 33 in thelight distribution curve 34.

In areas situated directly above the LED's the light distribution curve34 has a maximum (Lmax) while in the areas between the LED's it has aminimum (Lmin).

Since a specific quantity of light is necessary to expose thephotosensitive medium, Lmin will have to be at least such that thisspecific quantity is met. As a result, areas on the photosensitivemedium fed, for example, over broken line 30 along the row of LED's willreceive too much light.

FIG. 2a shows a number of imaginary strips 35, 36, and 37 of equal widthdx and situated in the auxiliary scanning direction 14. A small surfaceon the photosensitive medium having width dx passing strip 35 receiveslight, if LED 27 is energized, only during the period when said surfaceis situated opposite zone 38. An equally wide surface on thephotosensitive medium passing strip 36 receives a quantity of lightduring the period that LED 28 is energized and this surface is situatedopposite zone 39.

It will be apparent from the drawing that this is a maximum quantity oflight. A surface on the photosensitive medium passing the strip 37receives a minimum quantity of light. This surface receives light onlywhen it is situated opposite the zone 41 and then when it is situatedopposite zone 40.

Accordingly, the amount of light that a surface on the photosensitivemedium receives during the passage of said surface along the row ofLED's is proportional to the area of the traversed light-emittingsurface of the LED.

FIG. 3a is a diagram of an exposure device according to the invention. Arow of parallelogram-shaped LED's 45-49 is disposed on a support. EachLED is so constructed that the corner points 50, 51 of one short side ofan LED 46, as considered in the auxiliary scanning direction 14, are inline with the corner points 52, 53 of the other short side of aneighboring LED 47. Thus corner point 50 of LED 46 and corner point 52of LED 47 are on the same straight line 59 as considered in theauxiliary scanning direction, and corner point 51 of LED 46 and cornerpoint 53 of LED 47 are on the same straight line 60 as considered in theauxiliary scanning direction.

Here again, as in FIG. 2a, a number of imaginary strips 54, 55 are shownwith equal width (dx) and extending in the auxiliary scanning direction14. A surface on the photosensitive medium having a width dx passing thestrip 54 in the auxiliary scanning direction 14 receives light duringthe period that LED 48 is energized and said surface is situatedopposite the zone 56. A surface of equal width on the photosensitivemedium passing the strip 55 receives light during the period that LED 49is energized and said surface is situated opposite zone 58 and duringthe period that LED 48 is energized and said surface is situatedopposite zone 57.

As a result of the specific geometry of the light-emitting surfaces (theLED's) the area of zone 56 is equal to the sum of the areas of the zones57 and 58.

This relationship will apply to any arbitrary imaginary strip of widthdx so that each arbitrary surface in the exposed part of thephotosensitive medium will receive exactly the same amount of light. InFIG. 3b this equal light distribution is shown by curve 61. The quantityof light is equivalent to the minimum quantity Lmin in FIG. 2b.

The advantage of this equal light distribution is that the amount oflight required becomes as small as possible, so that the energizationcurrent becomes a minimum and overexposure of specific zones is avoided.

An additional advantage is that the influence of stray light is alsosmall and the exposure latitude increased.

Exposure latitude in this context denotes the ratio between the minimumquantity of light falling on the photosensitive medium locationsrequiring to be exposed (Lmin), and the quantity of stray light (Lst)that the photosensitive medium receives in those zones which are notexposed image-wise.

In the exposure device according to the invention, this ratio is at amaximum as a result of a distribution which is as uniform as possible.Other steps to make this ratio as large as possible are based on furtherreducing the quantity of stray light. Thus in an array of LED's thesurface or zone 15A (FIG. 1) surrounding the LED's which does not emitlight may be provided with a non-reflecting layer. The LED connectingleads, which are partially depicted in FIGS. 2a, 3a and 5 and designatedby reference character "L" frequently make a considerable contributionto stray light; Hence they may also be provided with a non-reflectinglayer, or be disposed so far away from the light-emitting surface thatthe reflections meet the Selfoc array 17 disposed above the LED's at toolarge an angle of incidence, so that these reflections are not imaged onthe photosensitive layer.

Another step to further reduce stray light is to dispose a diaphragm 62(FIGS. 1 and 3a) preferably having a substantially rectangular slitbetween the row of LED's and the Selfoc array 17. With approximately 5mm to 7 mm spacing between the LED's and the Selfoc array 17 a diaphragm62 having a slit width of of about 150 μm at a distance S (FIG. 1) ofabout 0.5 mm from the LED's is sufficient to reduce the stray lightconsiderably. The slit width d is a compromise between, on the one hand,the need to admit as much direct light as possible through the Selfocarray 17 and, on the other hand, the need to keep back as much aspossible that stray light which would meet the Selfoc array 17 at thecorrect angle of incidence. The distances between the diaphragm 62 andlight-emitting elements, and the slit width d of the diaphragm, asconsidered in a direction perpendicular to the row, is so selected thatrays of light which are emitted by the light-emitting elements (LED's)at an angle greater than the maximum angle of incidence β (FIG. 1) ofthe focussing glass fibres of aray 17 (about 24°) are held back by thediaphragm 62. Given a light-emitting element length c considered in thedirection perpendicular to the row, then the optimal relationshipbetween the diaphragm slit width d and the distance S measured from theLED to the diaphragm 62 for a maximum angle of incidence β of thefocussing glass fibres, is given by:

    d=c+S·tan β

In FIG. 3a, the distance b between two LED's 45,46 is equal to the widtha of an LED. Under the conditions drawn, the angle α is equal to 45°. Inthe embodiment illustrated, a=30 μm, b=30 μm. For the dimension c of anLED as considered perpendicular to the main scanning direction, c=60 μm.It will also be clear that any other dimension can be adapted tospecific needs.

An image that has to be depicted on a photosensitive medium consists ofa large number of image dots which must adjoin one another accurately inorder not to leave any unexposed portions. FIG. 4 shows a number ofimage dots on the photosensitive medium obtained with an exposure deviceaccording to FIG. 3a. The main scanning direction 29 is shown in thedrawing and the row of LED's (not shown) is situated directly abovethis. The photosensitive medium moves in the auxiliary scanningdirection 14.

The instant zone 65 is directly opposite LED 45 (FIG. 3a) LED 45 isenergized pulse-wise with a first image signal and thus exposes zone 65.Similarly zone 69 is exposed with LED 46 and zone 73 with LED 47. Theinstant that the photosensitive medium has been conveyed on over adistance of 1/2 c, or 30 μm, LED 45 is again energized with the samefirst image signal and thus exposes the zone 66. This zone 66 exactlyadjoins zone 65 as a result of the geometry chosen for the LED's.

Accordingly, LED's 46 and 47 expose the zones 70 and 74 with the firstimage signals for the LED's 46 and 47. After the photosensitive mediumhas again been conveyed over a distance 1/2 c=30 μm a second imagesignal is fed to LED 45. This second image signal energizes LED 45 sothat zone 67 is exposed. This zone again exactly adjoins zone 66. Afteranother movement of 1/2 c=30 μm LED 45 is again energized with the samesecond image signal and zone 68 is exposed. Similarly, zones 71 and 72are exposed by means of LED 46 with the same image signal and zones 75and 76 by means of LED 47.

An image line is a row of image dots situated adjacent one another asconsidered in the main scanning direction. An image dot is a zone on thephotosensitive medium obtained by exposing such zone with one and thesame image signal. This is shown in FIG. 4 by means of zones 77 and 78.The dimension of this image dot in the main scanning direction p andauxiliary scanning direction q is 90×90 μm.

As will also be seen from FIG. 4, all the exposed zones adjoin oneanother exactly in the main scanning direction and in the auxiliaryscanning direction so that there are no unexposed zones left.

FIG. 5 shows another embodiment of the exposure device according to theinvention. Here again the corner points 86, 87 of an LED 81 are in linewith the corner points 88, 89 of a neighboring LED 82. However, thedistance b between two LED's 80, 81 is exactly twice that of thedimension a of the LED's 80, 81. In the embodiment illustrated a=15 μm,b=30 μm and c=60 μm.

FIG. 6 illustrates a number of image dots on the photosensitive mediumobtained with an exposure device according to FIG. 5.

By analogy with the printing device shown in FIG. 4, the auxiliaryscanning direction is denoted by arrow 14 and the main scanningdirection and the position of the row of LED's by line 29. The instantthat zone 90 is directly opposite LED 80 (FIG. 5), LED 80 is energizedpulse-wise with a first image signal. When the photosensitive medium hasbeen conveyed on over a distance of 1/3c=20 μm LED 80 is again energizedwith the same first image signal and zone 91 is exposed. After thephotosensitive medium has again advanced 1/3c=20 μm, LED 80 is energizedfor the third time with the same first image signal and zone 92 isexposed. Each time that the photosensitive medium has moved 1/3c=20 μm,LED 80 is then energized three times with the next image signal so thatzones 93, 94 and 95 are exposed. In this way, a photosensitive medium isexposed to light accurately without any holes or overlaps.

The size of an image dot 96 is p×q or 60×100 μm. Thus many otherdimensions are possible to obtain accurate exposure of a photosensitivelayer, according to the idea of the invention. Provided that thecondition is satisfied to the effect that the corner points of twoadjacent parallelograms are situated perpendicularly above one another,exposure can always be obtained so as to fill the image completely.After each movement of the photosensitive medium over distance k, where:##EQU1## an LED must be energized in order to obtain complete filling ofthe image. Here, a is the width of an LED measured in the main scanningdirection, c is the dimension of an LED measured in the auxiliaryscanning direction and b is the distance between two adjacent LED's,also measured in the main scanning direction. D is a distance(preferably the largest distance) of which both a and b are completemultiples. If, for example, a b, then for the maximum value of D, D isequal to a or b. Alternatively, D may be made equal to 1/2 a or 1/3 afor example.

In an exposure device according to the invention the distance b betweentwo LED's can be selected to be smaller than the width a of an LED. Inorder to obtain complete filling of the image in this situation, an LEDmust just as well be energized after each movement of the photosensitivemedium over a distance k in accordance with the same formula. For a rowof LED's having the dimensions a=45 μm, b=15 μm, c=60 μm and D=15 μmapplies k=15 μm. In these circumstances each zone on the photosensitivemedium is exposed three times by an LED with pulse-wise energization ofthe LED's, while here again all the exposed zones receive exactly thesame amount of light. Since these zones overlap to some extent, someunsharpness occurs in this way at the edges of an image to be depicted.In the above described example, therefore, an adjoining strip of 15 μmwill be exposed twice at the edge of an image and a strip situatedbetween 15 μ m and 30 μm from the edge of an image will be exposed once.After development, the result is an edge which terminates via two greysteps, so that in particular, oblique lines on the photosensitive mediumhave a more uniform less sharply graduated appearance.

In the examples described it has always been assumed that an image dotis formed by energizing a light-emitting element a number of times withthe same image signal. It is equally possible, for example, with anexposure device according to FIG. 5, not to energize an LED 80 threetimes successively with the same image signal to form an image dot, but,for example, only twice so that only the zones 90 and 91 (FIG. 6) areexposed. Zone 92 remains unexposed. The result is an image dot of whichtwo-thirds is white and one-third is black. This is a form of image dotsize modulation with which a number of grey tints can be obtained.

The same technique is also applicable with light-emitting elementssatisfying the relation a>b, in which each image dot is obtained by anumber of overlapping exposures. Here again by applying for each imagedot less exposures (energizations) than is necessary for a completeexposure, image dots are formed with a specific grey tint.

However, it is always a requirement, for complete exposure of image dotsas considered in the main scanning direction, that such image dotsshould exactly adjoin one another so that a flat light distribution isobtained in the main scanning direction too, so that there are nounexposed zones left on the photosensitive medium.

FIG. 7 shows a further embodiment of a row of LED's. Light-emittingzones 98, 99 are disposed on a substrate 106. Electrodes 102, 105 arethen vapor-applied thereover to energize the light-emitting zones 98,99. As a result each LED comprises two partial zones 100, 101 and 103,104, respectively which are energized simultaneously when an imagesignal is applied. For each of these partial zones the corner points ofone short side of a partial zone as considered in the auxiliary scanningdirection must again be in line with the corner points of the othershort side of an adjacent partial zone. In the embodiment illustrated,the width of a partial zone is the electrode width, and the distancebetween two LED's is in each case 15 μm, while the length c of the LED'smeasured in the auxiliary scanning direction is 30 μm, so that appliesk=15 μm.

FIG. 8 shows an image dot formed with this array. The dimension in theauxiliary scanning direction is q=45 μm and in the main scanningdirection p=75 μm.

The description of the images on photosensitive medium is based on idealoptics and the use of pulses of very short duration for controlpurposes. However, the inventive idea can be applied equally to a systemin which the optics are not ideal and with any arbitrary pulse duration,an equal light distribution always being obtained.

The invention is not restricted to the embodiments described. The oneskilled in the art will be able to apply several variations thereto, allof which will however come under the following claims.

What is claimed is:
 1. An exposure device comprising at least onestraight row of light-emitting elements and an array of focussing glassfibers by means of which said light-emitting elements can be imaged on aphotosensitive medium, wherein a diaphragm having a slit ofpredetermined width is disposed between said array of focussing glassfibers and said light-emitting elements, the slit width of saiddiaphragm being substantially equivalent to a sum of a length of thelight-emitting elements measured in a direction perpendicular to saidrow and a product of a distance from the light-emitting elements to saiddiaphragm and a tangent of a maximum angle of incidence of the focussingglass fibers.
 2. An exposure device according to claim 1, wherein saiddistance from the light-emitting elements to said diaphragm isapproximately 0.5 mm and said slit width of said diaphragm isapproximately 150 μm.
 3. A printing device comprising an exposure devicehaving a plurality of light-emitting elements spaced apart from eachother in a row for image-wise exposure line by line of a photosensitivemedium advanced in an auxiliary scanning direction perpendicular to saidrow, each of said light-emitting elements being in a form of aparallelogram having long and short sides joined at corners, the cornersof one said short side of one of said elements in the auxiliary scanningdirection being aligned with the corners of an opposite short side of anadjacent element, said light-emitting elements being imaged on saidphotosensitive medium to form line by line a row of image dots thereon,and means for energizing said light-emitting elements in such mannerthat each zone receives a substantially equal amount of light on exposedareas thereof, wherein said energizing means energizes saidlight-emitting elements after each movement of said photosensitivemedium over a distance k, where ##EQU2## in which a is a width of alight-emitting element measured in a main scanning direction extendingsubstantially in the direction of said row, b is a distance between twoadjacent light-emitting elements in said main scanning direction, c is adimension of the light-emitting elements in the auxiliary scanningdirection, and D is a maximum distance of which both a and b are a wholemultiple.
 4. A printing device comprising an exposure device having aplurality of light-emitting elements spaced apart from each other in arow for image-wise exposure line by line of a photosensitive mediumadvanced in an auxiliary scanning direction perpendicular to said row,each of said light-emitting elements being in a form of a parallelogramhaving long and short sides joined at corners, the corners of one saidshort side of one of said elements in the auxiliary scanning directionbeing aligned with the corners of an opposite short side of an adjacentelement, said light-emitting elements being imaged on saidphotosensitive medium to form line by line a row of image dots thereon,and means for energizing said light-emitting elements in such mannerthat each zone receives a substantially equal amount of light on exposedareas thereof, wherein said energizing means energizes saidlight-emitting elements after each movement of said photosensitivemedium over a distance k, where ##EQU3## in which a is a width of alight-emitting element measured in a main scanning direction extendingsubstantially in the direction of said row, b is a distance between twoadjacent light-emitting elements in said main scanning direction, c is adimension of the light-emitting elements in the auxiliary scanningdirection, and D is a distance of which both a and b are a wholemultiple, wherein two light-emitting elements are energizedsimultaneously to form one image dot.
 5. A printing device comprising anexposure device having a plurality of light-emitting elements spacedapart from each other in a row for image-wise exposure line by line of aphotosensitive medium advanced in an auxiliary scanning directionperpendicular to said row, each of said light-emitting elements being ina form of a parallelogram having long and short sides joined at corners,the corners of one said short side of one of said elements in theauxiliary scanning direction being aligned with the corners of anopposite short side of an adjacent element, said light-emitting elementsbeing imaged on said photosensitive medium to form line by line a row ofimage dots thereon, and means for energizing said light-emittingelements in such manner that each zone receives a substantially equalamount of light on exposed areas thereof, wherein said energizing meansenergizes said light-emitting elements after each movement of saidphotosensitive medium over a distance k, where ##EQU4## in which a is awidth of a light-emitting element measured in a main scanning directionextending substantially in the direction of said row, b is a distancebetween two adjacent light-emitting elements in said main scanningdirection, c is a dimension of the light-emitting elements in theauxiliary scanning direction, and D is a maximum distance of which botha and b are a whole multiple, wherein two light-emitting elements areenergized simultaneously to form one image dot.